<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* .box{
            width: 0;
            height: 0;
            border: 10px solid transparent;
            border-bottom-color: #000;
        } */
        /* .box{
            width:100px;
            height: 100px;
            background-color: lightblue;
            margin:50px;
            position: relative;
        }
        .box::after{
            display: block;
            content: "";
            width: 0px;
            height: 0px;
            position: absolute;
            border: 10px solid transparent;
            border-bottom-color: lightblue;
            top: -20px;
            right: 15px;
        } */
        *{margin:0;padding:0}
        .box{
            width:100px;
            height: 100px;
            background-color: red;
            margin:50px;
            position: relative;
            box-shadow: 0 0 3px 2px rgba(51, 51, 51, 0.247);
        }
        .box::before,.box::after{
            content:"";
            display: block;
            width:30px;
            height: 30px;
            background-color: red;
            right:10px;
            top:-9px;
            position: absolute;
            transform: rotate(45deg);
           
            
        }
        .box::before{
            box-shadow: 0 0 3px 2px rgba(51, 51, 51, 0.247);
            z-index: -1;
        }
        .box::after{
            z-index: 3;
        }



    </style>
</head>
<body>
    <div class="box">
        
    </div>
 
</body>
</html>